computer science

Software package metrics

Software package metrics is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Software package metrics rather than just read about it. In short: Various software package metrics are used in modular programming. They have been mentioned by Robert Cecil Martin in his 2002 book Agile software development: principles, patterns, and practices.

Key takeaways

Software package metrics belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Software package metrics to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Software package metrics from memory before moving on to harder problems.

Reference excerpt

Various software package metrics are used in modular programming. They have been mentioned by Robert Cecil Martin in his 2002 book Agile software development: principles, patterns, and practices. The term software package here refers to a group of related classes in object-oriented programming.

Number of classes and interfaces: The number of concrete and abstract classes (and interfaces) in the package is an indicator of the extensibility of the package. Afferent couplings (Ca): The number of classes in other packages that depend upon classes within the package is an indicator of the package's responsibility. Afferent couplings signal inward. Efferent couplings (Ce): The number of classes in other packages that the classes in a package depend upon is an indicator of the package's dependence on externalities. Efferent couplings signal outward. Abstractness (A): The ratio of the number of abstract classes (and interfaces) in the analyzed package to the total number of classes in the analyzed package. The range for this metric is 0 to 1, with A=0 indicating a completely concrete package and A=1 indicating a completely abstract package. Instability (I): The ratio of efferent coupling (Ce) to total coupling (Ce + Ca) such that I = Ce / (Ce + Ca). This metric is an indicator of the package's resilience to change. The range for this metric is 0 to 1, with I=0 indicating a completely stable package and I=1 indicating a completely unstable package. Distance from the main sequence (D): The perpendicular distance of a package from the idealized line A + I = 1. D is calculated as D = | A + I - 1 |. This metric is an indicator of the package's balance between abstractness and stability. A package squarely on the main sequence is optimally balanced with respect to its abstractness and stability. Ideal packages are either completely abstract and stable (I=0, A=1) or completely concrete and unstable (I=1, A=0). The range for this metric is 0 to 1, with D=0 indicating a package that is coincident with the main sequence and D=1 indicating a package that is as far from the main sequence as possible. Package dependency cycles: Package dependency cycles are reported along with the hierarchical paths of packages participating in package dependency cycles.

See also Dependency inversion principle – a method to reduce coupling (Martin 2002:127).

References Robert Cecil Martin (2002). Agile Software Development: Principles, Patterns and Practices. Pearson Education. ISBN 0-13-597444-5.

External links OO Metrics tutorial explains package metrics with examples, but gets the Instability index wrong; see page 262 of Martin's Agile Software Development: Principles, Patterns and Practices. Pearson Education. ISBN 0-13-597444-5.

Worked examples

Example 1 — a first encounter with Software package metrics

Start with the simplest possible case. Write down what Software package metrics claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Software package metrics before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Software package metrics ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Software package metrics

In research: Software package metrics appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Software package metrics in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Software package metrics is common in secondary-school and first-year university syllabi. It links to neighbouring topics Object-oriented programming, Software metrics, so understanding it makes those chapters shorter.
In everyday life: Look for Software package metrics outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Software package metrics in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Software package metrics means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Software package metrics out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Software package metrics in simple terms?

Various software package metrics are used in modular programming. They have been mentioned by Robert Cecil Martin in his 2002 book Agile software development: principles, patterns, and practices.

Why does Software package metrics matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Software package metrics?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Software package metrics.